In optical communication systems, messages are transmitted by carrier waves of optical frequencies that are generated by sources such as lasers or light-emitting diodes. There is much current interest in such optical communication systems because they offer several advantages over conventional communication systems, such as a greatly increased number of channels of communication and the ability to use other materials besides expensive copper cables for transmitting messages. One such means for conducting or guiding waves of optical frequencies from one point to another is called an "optical waveguide". The operation of an optical waveguide is based on the fact that when a medium which is transparent to light is surrounded or otherwise bounded by another medium having a lower refractive index, light introduced along the inner medium's axis is totally reflected at the boundary with the surrounding medium, thus producing a guiding effect.
Certain electro-optical materials are very attractive for this application since they make it possible to achieve electrical control and high-speed operation of electro-optical devices and circuits. The use of lithium niobate (LiNbO.sub.3) crystals for such purposes is well-known in the art, and is disclosed, for example, in an article entitled "Integrated Optics and New Wave Phenomenon in Optical Waveguides," P. K. Tien, in Reviews of Modern Physics, Vol. 49, No. 2 (1977), pages 361-420. Lithium niobate has large electro-optic and acousto-optic coefficients and provides low loss propagation of light within waveguide layers or other regions within this material. Many different types of active channel waveguide devices using these materials have been used in a variety of electro-optical modulators and switches which are compatible with single-mode optical fibers.
Various methods of forming high refractive index waveguides in LiNbO.sub.3 have been used in the art. They include: epitaxial growth by sputtering, epitaxial growth by melting, lithium oxide (Li.sub.2 O) out-diffusion, and transition metal in-diffusion. Epitaxial growth by sputtering often leads to films with high losses and poor electro-optical properties. In epitaxial growth by melting, the film thickness cannot be easily controlled. The Li.sub.2 O out-diffusion process generates a film which can support only TE polarization waves (polarization parallel to the surface of the waveguide structure) propagating along the X axis on a Y-cut wafer.
The in-diffusion of a transition metal, such as titanium, nickel, or vanadium, into LiNbO.sub.3 offers a promising technique to produce planar as well as channel waveguide structures. The in-diffusion process involves evaporating a layer of metal, such as titanium, onto the surface of the crystal substrate by electron-beam sputtering techniques such as those described by K. L. Chopra, "Thin Film Phenomena," Chapter 2, McGraw-Hill Book Company, New York, 1969. The metal is then diffused at an elevated temperature, such as 900.degree. C., for an extended period of time (e.g., 6 hours). Typically, by this prior art method, a sample is cleaned and placed in a vacuum chamber which is evacuated to a pressure of 10.sup.-8 torr. Then, using an expensive and complex electron beam evaporation apparatus, titanium is evaporated onto the surface of the sample. These evacuation and evaporation procedures are time-consuming, requiring typically 4 hours to complete. Thus, the prior art process has the disadvantages of requiring expensive and complex apparatus, requiring the maintenance of a vacuum, and being time consuming.
In addition to the problems of implementing the above described prior art process, another serious problem arises because at the high temperature required for metal in-diffusion, loosely bound Li.sub.2 O diffuses out from the surface of the crystal structure. As a result of this Li.sub.2 O out-diffusion, a Li.sub.2 O-deficient planar waveguide layer is formed in the LiNbO.sub.3 crystal in addition to the waveguides formed by metal in-diffusion. The waveguide formed by out-diffusion can confine TE polarization waves propagating along the X-axis on a Y-cut wafer (or the Y-axis on an X-cut wafer) in an undesirable manner. (A Y-cut wafer is a wafer cut perpendicular to the Y-axis of the crystal. For a more detailed description of crystal cutting, refer to "Standards on Piezoelectric Crystals, 1949," Proceedings of the Institute of Radio Engineers, pp. 1378-1395, December 1949). In a channel wave-guide device, a planar out-diffusion waveguide introduces excessive cross-talk between guided modes from two adjacent waveguides. Cross-talk presents particular difficulties when trying to achieve compatibility between a fiber optic communications link and optical channel waveguide switches based on controlled coherent coupling. The planar index increase caused by the out-diffusion of Li.sub.2 O limits the implementation of the coherent coupling switches to TM waveguide modes only (i.e., polarization perpendicular to the surface of the waveguide structure). In addition, in an end-butt coupling configuration between a single mode optical fiber and a channel waveguide, a large portion of the optical energy goes to the unwanted out-diffusion modes, which are readily excited by the optical fiber input, and thus the coupling to the channel waveguide is effectively diminished.
The cause of the out-diffusion of Li.sub.2 O from LiNbO.sub.3 crystals is inherent in the particular structure of these crystals. It is well known that LiNbO.sub.3 crystals can be grown in a slightly non-stoichiometric form, (Li.sub.2 O).sub.v (Nb.sub.2 O.sub.5).sub.1-v where v ranges from 0.48 to 0.50. At the high temperature (850.degree. C. to 1200.degree. C.) required for the in-diffusion of transition metal ions in order to form a waveguide in LiNbO.sub.3 crystals, the loosely bound Li.sub.2 O diffuses out from the surface of the crystal. It is known experimentally that for a small change of v in LiNbO.sub.3, the ordinary refractive index remains unchanged while the extraordinary refractive index increases approximately linearly as v decreases. The reduction in the Li.sub.2 O concentration at the surface of the crystal due to out-diffusion thus forms a high-index layer which traps optical beams in the direction perpendicular to the surface of the waveguide structure.
One method for suppressing the out-diffusion of Li.sub.2 O from LiNbO.sub.3 and LiTaO.sub.3 waveguide structures is disclosed in U.S. Pat. 4,196,963, assigned to the present assignee and includes exposing the LiTaO.sub.3 and LiNbO.sub.3 crystal structures to a Li.sub.2 O-rich environment at sufficient vapor pressure that Li.sub.2 O diffuses into the structure as a compensation process and a solid-solid surface interaction occurs. The Li.sub.2 O-rich environment is obtained by annealing the structure in a high purity powder of LiNbO.sub.3 or LiTaO.sub.3 or in molten LiNO.sub.3.
The invention described in this copending application is highly effective in suppressing Li.sub.2 O out-diffusion from LiNbO.sub.3 and LiTaO.sub.3 crystals. However, the present invention provides still other novel and alternative means for preventing Li.sub.2 O out-diffusion in selected waveguide materials. In addition, the present invention provides an improved method for diffusing a metal into a substrate to form a waveguide structure, which overcomes many of the disadvantages of certain prior art processes.